Going by his old posts, it's a 2 seater.
Try contacting Warren @ AllZParts in Sydney - last time I spoke to him he had a couple of rear 1/4 panels available.
You should need to replace the rear of the body shell, unless it REALLY rusty.
You'd be better off unpicking the damaged panels and panel beating them straight...

Evan, if you are the guy who messaged me on Facebook a few weeks ago about repairing it, I replied to your message.
Regardless, give me a call on: Zero Four Zero Zero Zero Five Four Five Seven Two
I'll be able to fix repair it

Nathan is on a round Australia trip with his family - don't expect an answer any time in the near future...
And don't be an idiot - a set of rusty chassis rails isn't going to scrap a Z!
They are not hard to fix:

I've not seen that before!
Rear Counter-shaft nut appears to have come loose, and that almost looks like the rear CS bearing poking through
It would have just walked 5th CS gear off the back of the shaft, the nut has dropped down into the bottom of the housing.
So my guess would be to replace the extension housing, bearing and nut.
Make sure the rear CS nut has Loctite applied to it during reassembly...
Take pictures of the disassembly!